The Gen. 3 aluminum block LM4 engine came in Chevy, GMC, Buick and even Isuzu SUVs from 2003-05.

It was known as the Vortec 5300 and made a little less power than its aluminum-block brother, the L33. This was due to the LM4’s smaller intake valve Cathedral port heads, lower compression, and a slightly smaller cam. With the exception of the block material, it had more in common with the iron-block LM7.

The LM4 was built during the transition between Gen. 3 and Gen. 4 truck engines. Because of this, the ’03 model had three different head bolt lengths, early-style cam housing bore and a return-style fuel system.

The 2005 model was unique in that it had the beefier Gen. 4-style full-floating rods. If the engine was used in a 4WD SUV, the oil pan had a tunnel through the middle to accommodate the front axle.

If you are looking into an engine swap, it’s not a bad candidate because the oil pan can be easily switched out, but be aware that the blocks may or may not have the dipstick tube location drilled. Sometimes it’s merely plugged, but it’s possible you’ll have to drill it yourself.

They can be found for less than $1,000, which is inexpensive for an aluminum block engine.

Whether you’re towing on low-octane or springing for good fuel, a tuner can dial-in the ECM and take it to the next level. Raising the factory redline is a big part of this because it allows your vehicle to be in the meat of the powerband longer.

Torque limiting can be completely shut off, shift points can be raised, and it makes it easier to tune for a bigger cam and injectors later on.

Before beginning the tuning process, we recommend installing a colder thermostat to open up the tuning window.

Below are some more upgrades you can make to improve the performance of an LM4.

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Upgrading the LM4 Engine Camshaft and Valvetrain

An LS3 or LS9 cam makes good power, but not where you want it if you’re using the LM4 engine in a truck. What you need is a cam that delivers a gut-punch right when the converter hits. We recommend a dedicated truck cam.

To maximize torque in the mid-range in a truck cam, manufactures close the intake valve at about 35-40 degrees (@.050 in.) after bottom dead center and alter the intake valve opening to set the idle quality.

What if you already have a power adder?

Generally, supercharger cams and nitrous cams will have slightly more lobe separation and longer exhaust duration. Turbo cams reduce overlap with less exhaust duration split in relation to the intake.

The stock rockers are good up to 175 lbs. of seat pressure and 450 lbs. open. You will want to install a trunnion kit for added reliability.

Intake Duration (@ 0.050 in.)

Horsepower at the wheels after bolt-ons

Idle Quality

Notes

191° (Stock)

235-245 whp

Smooth

Heavy drivetrain.

210°

+50 hp

Slightly noticeable

Good with auto and stock converter.

215° - 225°

+75 hp

Steady lope

Converter recommended. Still can drive daily.

225° - 230°

+100 hp

Lopey

Heads and intake good for another 50+ hp.

Drop-in .500-in. lift cams are popular, but LS6 springs allow you to run .550-in. lift and extend the rpm range. Spring life isn’t a problem because trucks generally don’t spend a lot of time at high rpm. Beyond that, .575 to .600 in. isn’t a problem with dual valve springs.

The stock rockers are good up to 175 lbs. of seat pressure and 450 lbs. open. You will want to install a trunnion kit for added reliability.

Upgrading the LM4 Engine Fuel System and Tuning

(Image/LS1Tech)

Tuning the computer changes the fuel and ignition curves to increase performance. Plug-in programmers are easy to use but have limits.

If you plan on running a power adder or have a long list of upgrades planned, it’s recommended that you take your truck to a chassis dyno tuner who can give you a custom tune. Knowing you’re running high octane and having the before-mentioned 160-degree thermostat opens up the tuning window.

Depending on how much power you intend to make, upgrading to larger fuel injectors may be needed. Ask your tuner which injectors they prefer and have them ready to save yourself an extra trip to the dyno.

The Mini-Delphi/Multec II connector 25-pound injectors are a limitation at around 380 whp. If you upgrade, you may need a fuel injector adapter harness, depending on which injectors you choose.

The factory fuel pump will become a limitation around 430 whp, so you should plan on upgrading the fuel pump as well.

When running boost, you can use a water-methanol system to supply extra fuel and lower charge air temps.

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Upgrading the LM4 Engine Intake Manifold and Throttle Body

The LM4 intake wasn’t particularly good and had a 3-bolt 78mm DBW throttle body.

A better option is the Chevy Trailblazer SS or the 2007.5+ “NNBS” truck intake. They are the best of the factory manifolds with a 4-bolt 87mm throttle body, bigger standard EV6 style injectors, and will pick up about 15 horsepower.

Keep in mind, the original throttle body had an 8-pin connector.

To run the newer style 6-pin throttle body requires a Torque Rush X-Link harness adapter. You will need to re-route the EVAP line and may have to extend the MAP sensor harness.

[Trying to find an LS engine for a swap or build? Check out Part 1 and Part 2 of our LS Spotter’s Guide.]

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Upgrading LM4 Engine Cylinder Heads

The heads used on the LM4 had small 1.890 in. intake valves that cap intake flow around 225 cfm.

If you’re looking for 400whp, you can achieve it with good drivability and power across the entire rpm range by upgrading the cylinder heads.

Aftermarket heads reduce downtime and are cost-effective when you sell your old heads in good condition. The decks are thicker to withstand more boost.

A 205cc runner head is a good match with the TBSS or 2007.5+ intakes. They easily pick up another 50 cfm of flow. The combination of the two will pull strongly up to 7,000 rpm.

Don’t mind a little downtime?

The 862 and 706 casting Cathedral-port cylinder heads can be CNC-ported to flow close to the aftermarket heads. With a bigger cam you can mill the heads for a little over 10:1 compression which runs on pump gas. If you are going through the heads, it’s recommended to switch to the light hollow-stem intake valves from an LS3. They can be cut to 2.000-in. to fit the standard valve seats and extend the rpm range up to 7,000 rpm.

Keep in mind, if you’re running a cam in the 230 range, piston to valve clearance needs to be checked and fly-cutting may be required.

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Adding Boost to an LM4 Engine

(Image/Bozi Tatarevic – Hoonable.com)

Although an iron block 5.3L will handle more power, the LM4 can still support over 850 whp.

You’ve likely upgraded the fuel system and injectors if you’re looking at power adders.

Remember, a 4-corner steam kit reduces hot spots that cause the rings to butt and snap the piston’s ring lands.

A nitrous oxide kit(at low settings) is great for street driving with stock internals. Up to a 200-shot is common. Keep in mind the tight piston ring gap is the limiting factor beyond that. If you’re wanting to get serious, a single-plane intake is less prone to break from a nitrous backfire. A plate system has better distribution than the original intake, but an eight-nozzle fogger system is even better. Running higher-octane fuel is advised.

Truck engine bays make fitting turbos easy. Single turbo systems using turbo exhaust manifolds are an inexpensive way to make big power. If you’re running a single turbo, the T4 hot-side fits well, but the small turbine diameters limit exhaust flow. The 650 whp begins to feel like 400 did in a hurry, so take this into consideration. V-band style exhaust housings are available with larger turbines and make plumbing easier. Although twins are a little more expensive out of the box, you’ll have more room to grow.

A Roots-style supercharger is dependable and makes great torque in the low- and mid-rpm range. It’s great for melting tires.

A centrifugal-style supercharger is lightweight and makes more power at high rpm. This is partially due to a larger intercooler mounted in front of the radiator.

Still looking for more? Let’s talk about bottom-end upgrades.

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Upgrading the LM4 Engine Rotating Assembly

The dished LM4 pistons net 9.5:1 compression.

That’s a little low for a naturally aspirated engine, but on-target if you’re adding boost.

The problem being the pistons are cast and the rings butt when you start making power.

A set of forged pistons should be high on your priority list. They have stronger wristpins, thicker ring lands, and the added valve reliefs allow you to run big cams. If you’re going over 800 hp, a set of .200 in. wall tool-steel pins is a good idea.

The first two years of the LM4 had Gen. 3 rods and they are a little dicey. They can handle about 650+ hp and 7,000 rpm in boosted applications (at least for a while). They are likely to bend before they break when subjected to real track conditions.

If you’re getting forged pistons, upgrade to forged connecting rods at the same time. Big 7/16-inch rod bolts will go a long way to keep things together above 7,000 rpm.

The LM4 crank was cast but strong. It’s been known to handle over 1,000 whp.

The main reason for going with a stroker forged crank is for the added cubic inches. The extra cubes bring boost on quicker which means you can use bigger and more-efficient turbos.

Upgrading the LM4 Engine Block

(Image/LS1Tech)

The LM4 engine block has a 3.780-inch bore diameter. The cylinders can be bored to the LS1/LS6’s 3.905 inches, but it’s recommended that you leave them as thick as possible with common .020 /.030 / .040 in. oversize forged pistons if you are boosting it.

You can make 700+ hp with a couple of simple upgrades like head studs and LS9 head gaskets.

The head gasket bore diameter isn’t optimal, but with careful tuning and race fuel it can withstand 850+ whp. It’s better to reduce ignition timing and compensate with added boost to reduce the cylinder pressure spikes that lift heads and cause the main caps to dance.

The factory main caps aren’t doweled. Main studs should be added any time you’re this deep in the engine.

Something to keep in mind: The 2004-and-up engines (even Gen. 3) had two head bolt lengths vs. three on the earlier engines.

Happy building.

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(Information for this article originally appeared in thisUpgrading the Gen. 3 5.3L, Aluminum Block, LS Truck Enginearticle atSummit Racing’s searchable database of FAQ tech info. Go there and search “LS engines” for a comprehensive collection of LS engine tech information.)

Author: Brian Nutter
After a stint in the U.S. Air Force, Brian Nutter studied at the Houston, TX-based School of Automotive Machinists in 1997. The early part of his automotive career included working for engine builders Scott Shafiroff and C.J. Batten, followed by several years developing performance pistons at Wiseco Piston Co. Today, Brian develops performance parts for Summit Racing Equipment and is a regular OnAllCylinders contributor. For fun, he runs his 427-powered C5 Z06 in ECTA land-speed racing, at OPTIMA® street car events, and at a mix of autocross, drag racing, and track days.

Hi DJ, the 243’s (or 799’s with their similar chamber and flow) would be excellent upgrades.

The 243’s are a bit better to start with their hollow intake valves.

Another good option for either is to cut LS3 intake valves down to 2.000 in. so it fits the factory valve seat. They are very light and allow the engine to rev even higher before valve float.

Flow up to 323 cfm can be achieved with some of the porting programs from Total Engine Airflow and others as measured on a 3.900 test bore.

Forged flat tops will have valve reliefs in the 2 or 3 cc range. The valve reliefs low for larger cams and the forged construction is great for power adders. Along with the bigger 65cc combustion chambers, they would net 9.8:1 compression, which is great for boost.

If you are staying naturally aspirated or nitrous, mill the heads down .030 in. They will still have a good fit to the intake manifold, drops to 60cc and 10.5:1 is the result.